Where a pipe is joined to plastic, e.g., polyethylene fittings, couplings, structures or other pipe by an elastomeric gasket, such as in a bell and spigot joint, considerable interference between the spigot (or pipe) outer surface and the inner surface of the plastic bell is required to effect a water tight or low pressure resistant seal. Because of this interference, a large measure of force is required to assemble the joint. This joining force develops considerable ring tensile strain in the plastic bell. The bell can then exert enough ring compression on the spigot so that when the joining force is released, the spigot will be forced or "squeezed" back out of the bell, and the joint will come apart. This problem, which is sometimes referred to as "back out", is magnified when both the bell and spigot are made of plastic.
One attempt to overcome this problem has been to use a lubricant on the bell surface and the gasket in an effort to minimize the joining force required to join the bell and spigot. However, the use of lubricant in this way only exacerbates the problem, since it also lowers the amount of ring compression required to force the spigot back out of the bell.
A common joint for plastic pipe, couplings and fittings involves the use of an O-ring held in an annular groove near the end of the pipe. The end of the pipe with the O-ring on it is inserted into a bell attached to a fitting, coupling or other piping structure. However, this jointing system suffers from the "back out" problem discussed above. Another method for joining plastic pipe involves the use of a shoulder gasket at the end of the pipe. This jointing assembly is particularly affected by the "back out" problem because the ring compression force is applied to the very end of the pipe, i.e., where the gasket is in contact with the inner surface of the bell.
A plastic pipe joining system has now been discovered which overcomes the "back out" problem described above, while at the same time permitting the use of a shoulder gasket of the end of the pipe.